1. Field of the Invention
The present invention relates generally to a system and method for controlling an engine wherein a value correlative to an output generated from the engine is detected and an engine throttle valve, an opening angle of which determines an intake air quantity, is actuated so that the detected value reaches a target value, wherein in addition to the actuation of the throttle valve to control intake air quantity an air-fuel mixture ratio is controlled so that the engine output reaches near a limit value in a stable region previously set, and wherein in addition to the actuation of the throttle valve to control intake air quantity an ignition timing is so controlled that the engine output reaches maximum.
2. Description of the Prior Art
In a conventional gasoline engine, an accelerator pedal is mechanically linked with a throttle valve installed within an engine intake manifold. An angular position of the accelerator pedal (when an operator depresses the pedal) determines an opening angle of the throttle valve, i.e., an intake air quantity supplied to the engine so that the engine output is directly controlled.
In details, in a conventional electronically controlled engine, an optimum quantity of fuel injected into the engine is previously set according to the intake air quantity thus determined, an engine speed, various engine operating parameters such as engine cooling temperature. A plurality of fuel injection valves each located within a corresponding intake port of the corresponding engine cylinder which is located downstream of the throttle valve inject fuel a quantity of which is determined by the optimum quantity of fuel previously set, so that a mixture of fuel with intake air is formed and supplied to the engine body, thus the engine operation being controlled.
Since in such conventional engine control systems the quantity of fuel supplied to the engine is controlled on the basis of the actual quantity of intake air determined according to the opening angle of the throttle valve, i.e., the angular position of the accelerator pedal, the engine output is varied even though the same throttle valve opening angle depending on the difference of an environment under which the engine is placed (, e.g., atmospheric pressure and surrounding air temperature of the engine).
For example, when the above-described engine is placed at a high ground, i.e., under a low atmospheric pressure, a substantive intake air quantity is reduced so that the quantity of fuel injected into the engine is accordingly reduced, thus the engine output being reduced, even though the opening angle of the throttle valve is the same as that when the engine is placed at a low ground, i.e., under a normal atmospheric pressure.
Therefore, it is necessary to depress the accelerator pedal through an angle greater than that when the engine is placed at the low ground in order to achieve the same engine output as that when the engine is placed at the low round. In this way, an engine driveability will be lowered.
In the similar way, the variation in the engine output is caused by aging effect even though the opening angle of the throttle valve is the same so that the engine driveability will be lowered.
Furthermore, since a deviation of the engine output from a target value means that the engine output is deviated from a stable region in which the engine carries out a stable operation, the engine cannot provide a sufficient engine output and it results in a wasteful consumption of fuel when the engine output is out of the stable region.